1. Field of the Invention
The present invention relates to a device for detecting a transmissivity of a substance by utilizing a transmitted light value, and more specifically, relates to a device for detecting a degree of contamination of a fluid, for example, a gas or a liquid.
2. Description of the Related Art
In many industrial fields a check of the transmissivity of a substance is made, during and after manufacture, and further, upon use thereof, by using a transmitted light value.
For example, when mixing a plurality of substances such as gases or liquids to produce a different substance, or when blending certain ingredients, for example, inorganic particles such as carbon, ferrite, antimony oxide or dyestuff into a base material such as water, paste or resin, a continuous check of the transmissivity of each material used or produced in the respective process must be made by using a detecting system utilizing a transmitted light value system, to ensure that the product has a predetermined condition, including the quality or contamination thereof, or to maintain the processing conditions at the optimum state.
Further, a level of contamination of a substance such as a gas or liquid used in a certain process must be checked to carry out a process control, or to determine when the substance must be discharged before causing environmental pollution. For example, a degree of contamination of a lubricating oil of an internal combustion engine by carbon particles contained therein must be checked.
Generally speaking, a check of a transmissivity of a substance, i.e., the quality or contamination thereof, is made by using a transmitted light value, which is represented by a ratio of an intensity of an incident light to an intensity of a transmitted light when a light is incident on a substance such as a liquid or a gas having particles therein. This light transmittance is defined by the following equation. EQU .alpha.=1/l.times.ln I.sub.0 /I
Wherein, .alpha. represents a light transmittance which relates to a function representing a condition including a quality or level of contamination of a substance, l represents a thickness of an article, and I.sub.0 and I represent an intensity of an incident light and an intensity of a transmitted light, respectively.
Therefore, by arranging a light emitting element and a photo detecting element at a predetermined space therebetween, and placing a substance to be checked in that space, a light transmittance value depending upon a condition of the substance can be obtained from the output of the photo detecting element.
However, in the light transmittance representing a certain condition of a substance by such a detecting system, as apparent from the equation above, the output of the photo detecting element becomes extremely low because the strength of the transmitted light is logarithmically reduced as the degree of contamination is increased.
Accordingly, problems arise in that it is difficult to transmit such a weak output signal to a display means provided, for example, at a location remote from the detecting device, and thus the transmitted signal must be amplified, which means that an amplifier capable of a large amplification must be provided in the vicinity of the photo detecting element.
Further, other problems arise in this type of detecting system, such as another circuit, for example, a comparator for comparing the transmitted light with the incident light or a compensating circuit for compensating for variations in the amount of light emitted from the light emitting element, must be provided, thus increasing the size of the detector system and making the configuration thereof more complex.
Also, as discussed hereafter, another problem will arise when such a system is used for detecting a degree of contamination of a lubricating oil of an internal combustion engine, for example, a diesel engine. In this system, a degree of contamination of the lubricating oil by carbon particles is usually measured.
Generally speaking, a lubricating oil of an internal combustion engine should be changed periodically to avoid an increase of the wear of slidable parts of the engine caused by contaminated oil.
The amount of contamination of the lubricating oil differs significantly in accordance with the operating conditions of the engine, but the periods at which the lubricating oil changed are usually based on the length of time for which the engine has been operated or the distance that the vehicle has been driven.
Accordingly, problems arise in that the lubricating oil is wasted by changing it too early or wear of the sliding portions of the engine is greatly increased because the oil is not changed before the degree of contamination thereof becomes too high.
In a diesel internal combustion engine in particular, a large number of carbon particles contained in the exhaust gas are mixed in the lubricating oil, and therefore, the problems mentioned above become more important because the lubricating oil is contaminated in a relatively short time when compared with an internal combustion engine using gasoline.
To overcome these problems, Japanese Unexamined Patent Publication 57-98842 disclosed a system in which a degree of contamination is detected by detecting a degree of a transparency of the lubricating oil when placed in the space between the light emitting element and the photo detecting element, both of which are immersed in the lubricating oil, and a display lamp is turned ON when a signal is output at a certain degree of contamination of the lubricating oil, to signify that the oil must be changed.
But if such a system is used to detect a degree of contamination value of a lubricating oil in a diesel internal combustion engine, for example, since the light emitting element and photo detecting element must be immersed in the lubricating oil, the temperature of which becomes as high as 145.degree. C., the light emitting element and the photo detecting element must be able to operate at such a high temperature, and such elements having a high heat resistance are not easily obtained commercially.
In experiments with a diesel internal combustion engine using a light emitting diode (LED) as a light emitting element and a silicone photo diode (SPD) as a photo detecting element, it was found that the output current of said photo detecting element was extremely low, i.e., a micro current of about 1 nA, and because of the very low output current, it was necessary to provide an amplifier or the like near the photo detecting element. However, difficulties arose in the operation of such an amplifier at a temperature as high as 145.degree. C.